Month: January 2013

I have recently learned from my island friend Madalena Patacho of Bom Bom Island that a Cape fur seal (Arctocephalus pusillus) was seen and photographed on a beach at the north end of Príncipe Island and was also later seen by a number of islanders along the São Tomé coast.

Why is this noteworthy? It is special because although vagrant individuals of this large species have occasionally been seen as far north as the southern border of Angola, the nearest breeding colony of Cape (or South African) fur seals is just under 2,700 km south of Príncipe, at Cape Cross Namibia! This is almost exactly the same distance as between Príncipe and Kampala, Uganda.

My colleague, Bastien Loloum works for MARAPA, an NGO on the islands heavily involved in marine life and conservation. Bastien is coordinator of a cetacean monitoring program there. So far they have recorded ten species of cetaceans but he informs me that there are no formal records of the Cape fur seal in the islands waters. However, he and his colleagues think that this is may be the second or third account of a vagrant fur seal there during the past decade. This would be rare indeed, but for reasons given below, I am not too surprised.

Map from IUCN Red list.

The Cape or South African fur seal is the largest of its kind (Otariidae= fur seals + sea lions; true seals lack external ears and belong to a different family, the Phocidae); males reach 2.27m in body length and 360kg in mass. Females are much smaller. As the map shows, this species lives and breeds along the South African and Namibian coast. There are about 23 major breeding colonies, and there are population estimates of over one million individuals. In 1992, I was fortunate to explore the Skeleton Coast of Namibia by air from Capetown to about 16º south latitude, and the Cape fur seal colonies were impressive to say the least.

Above the Namib coast; RCD photo 1992

Above the Namib coast; RCD photo 1992

Later and somewhat farther north, we were able to approach a colony by land as there were intervening rocks disguising our presence.

On the Namib Coast. RCD phot. 1992

On the Namib Coast. RCD phot. 1992

Note the conspicuous external ear which is one characteristic that differentiates these mammals from true seals. RCD phot. 1992

The key to understanding how these enormous populations of large pinnipeds are sustained, and how one individual might have arrived in the Gulf of Guinea 2700 km to the north lies in understanding the nature of the Benguela Current which flows up the west coast of southern Africa. The Benguela is a cold current and is thus highly oxygenated water. This supports vast amounts of plankton which in turn provide sustenance for a huge marine fauna “higher up the chain.” In fact, the Benguela Current is one of the richest fisheries in the world.

Notice the persistent, thick, slightly discolored foam on the shore which indicates high plankton content in the water. RCD phot. 1992

A major factor important to our seal story and indeed, perhaps to the early colonization of the Gulf of Guinea islands’ unique plants and animals, is the fact that the Benguela Current flows from South to North and has undoubtedly done so since the Atlantic Ocean opened up in the Cretaceous.

Dominant currents in the Gulf of Guinea and South Africa. CAS construct

As the map shows, the Benguela flows north past the Congo River Delta. At the same time, the Guinea Current flows West to East across the Niger Delta. These two currents converge in the Bight of Benin to form the South Equatorial Current, and this major current flows due West, directly through the Gulf of Guinea Islands and across the Atlantic.

Because these two major currents cross the deltas of two of the mightiest rivers in Africa each with huge interior drainages, then change direction westward through São Tomé and Príncipe suggested to us that many of the endemic plants and animals on the two islands arrived there millions of years ago by rafting. We suggested that the raft(s) would be large chunks of riverbank which broke off and floated to the sea. Our hypothesis that the rafts would have been very large is supported by the fact that a significant percentage of the unique reptiles (and one amphibian) of both islands are burrowing species (see inset, above), unlikely to cross ocean barriers on small floating objects. We published this hypothesis in the Journal of Biogeography in 2007.

With respect to our Cape fur seal visitor, a closer look at the first image reveals that this animal ran afoul of a fishing operation of some sort; note the blue polyethylene rope or netting around its neck which may have come from a trawling operation to the south. While it is highly unlikely that the poor creature was dragged 2700km, it IS possible that having been entangled, and then escaping, the seal became disoriented and probably very much weakened. It seems likely that the “line of least resistance” would be to follow the Benguela Current until it converged with the South Equatorial Current, ultimately depositing the animal in the waters off São Tomé and Príncipe. This, in a sense, would be following part of the same dispersal pathway as the original plant and animal colonizers of the islands if our rafting hypothesis is correct. It is impossible to know for sure, but this seems to me a possible scenario.